top speed 114mph, What the F&*(?

[MindWebs]

i have an engineering question...

why can both of my 250cc bikes do 108mph

and my 1100cc bike only runs 114?

this make no sense to me.....
If someone would kindly explain why a 1100cc machine falls flat on it face at
114mph when if i swaped sprockets i could "go" faster on my 250.

as a side note.. Yes, i'm planning the FD swap(XJ750 FD).

I'm running a 79 xs11 special, Did i mess something up? Or is this true top speed for my machine?

most bikes around 500cc can break 120, most bikes 650 and up can do 130 min. (bikes newer then 1983)

what's the deal?

[FJ111200]

One theory from the UK site,

Re: Fuel Tank capacity.
« Reply #29 on: Today at 07:58:30 AM »

--------------------------------------------------------------------------------
Unfortunately it's the way of the motor industry, they make the first off, a superb beast and then gradually detune and sanitise the machine for the crowd that couldn't hack the proper ones. They then, through clever marketing, convince the saddos (no offence) that what they are geting is better. Oh how easily people are fooled.

Datsun did it with the Z series
Honda did it with the CBX
Yamaha did it with the XS11 (but went toooo far)

.................................................. .................................................. ...

or maybe it's just shagged and in need of a good overhaul.

[trbig]

I would say you have some carb or exhaust issues if you only get 114. I had a cracked exhaust recently and had a top end of @ 118 per GPS. I fixed that and did nothing else and hit 126 without trying too hard.

I don't remember the exact formula, but after you start pushing winds at a certain speed, you start needing exponentially more HP with equal wind drag. Something like a bike that does 140, needs nearly double the hp to do 150.. or something like that. BUT... a bigger motor will get you to that point much QUICKER... if we're talking 4 stroke motor versus 4 stroke.


Tod

[TADracer]

If everything is in good mechanical shape such as cams timed well, good compression, adequate spark then it has to be tuneup related.

A 79 special with the 16" rear wheel numerically should do 170 or so in 5th at the 8500 rpm redline but like trbig says, over 130 or so, power consumption vs speed gained becomes an issue and things will flatten out quickly. In my younger and wilder days I have been over 130 on mine several times but it took some careful tuning to get there.

[trbig]

A 79 special with the 16" rear wheel numerically should do 170

I'm assuming a typo here... that should read 140.


Tod

[Wildkat]

lol
That's what I figured too...
I've done 125 with more throttle to go... but if I thought the XS could do 170... I wouldn't have much need for the FJ...

[danteinien]

Your XS is most likely not putting all the power to the ground that it could

I've hit 125, and at the rate it was going 135 would have happened about 3 seconds later....darn highway patrol...

[DJinNH]

Your XS is most likely not putting all the power to the ground that it could

I've hit 125, and at the rate it was going 135 would have happened about 3 seconds later....darn highway patrol...

Better the highway patrol slowing you down than a guardrail.

Don

[Bigfoot]

I have had the XJ and XS in the mid 120's and was still accelerating. Both bikes were pretty much stock.

[WMarshy]

Better the highway patrol slowing you down than a guardrail.

Don

I think we all need reminding of that once in a while (maybe just me?). Last night the nice officer reminded me. Seems every time I step out of line they are their to remind me that I should be "fill in lecture here". I guess I picked the wrong time/area to check top end... Back to reality now, no more stupid shazit, gotta stop and think before following that dumb idea…
I did shake the guys hand and thanked him for being kind.

[Ivan]

I haven't run the stock FD for quite a while, but if I am really pulling, I am still in fourth at 115.

If you want the speed, make sure you keep it wrapped up. If you shift and it goes under about 5k, you're not going to win that battle.

[Crazcnuk]

A lot of it comes down to the engineering in both bikes.

An XS1100 is about 95hp, in perfect tune, but wieghs 630lbs.

A 2006 cr250 dirt bike is 59hp @ 212lbs.

The difference is the xs can sustain the high speed all day, whereas the dirt bike would not last very long at full speed on a highway.

[webbcraft2150]

I think you need to get into that motor some. My 80g hardbagger w/full Windjammer has tagged 117 on the speedo with two up and it was still pulling. A observed 121 was hit riding solo before the killer wobble set in. So that being said, and the other testements to high triple diget speeds, ya might wanna start with the basics. Pull the plugs and read em, that will give you some indication how the carbs are working. First thing I would do would be a compression check to see how healthy the basic motor is. Trying to tune a tired engine can be pretty much fruitless. Aerodynamics play a big part when you start clearing a 100mph but even the upright position of a special should net you a higher top end than 114. As mentioned before, you need to keep the motor wound up pretty tight in 3rd, 4th and keep it twisted tight in 5th. Fuel flow was also mentioned, if there ain't nuff gas flowing, its not gonna go very fast and may "seem" like its running right.

[crazy steve]

Top speed is all about total power; gearing really doesn't enter into it (unless the vehicle will redline in top gear) as long as you're reasonably close to the torque peak at maximum speed. Most vehicles will 'calculate' out to a much higher top speed than they can actually attain. As an example, my XJS Jag was advertised as having a 160 mph top speed, but this was at 5K rpm. Redline on the motor is 6500 and the calculated top speed is over 190.

With that said, you're definitely down on power, as any XS11 should be able to break 120 with ease. For what it's worth, I saw 141 indicated on my '78 (just once! honest!) when it was fairly new.

And a guy I know who runs at Bonneville told me that for every 10% increase in speed, you need 30% more HP, so the numbers get pretty big pretty fast.

'78E original owner

[yamahansolo]

you have to turn the throttle all the way...